When preprocessing, handle directives, but do not expand macros.
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
-@@ -11863,6 +11863,12 @@ by @option{-fplugin=@var{name}} instead
+@@ -11865,6 +11865,12 @@ by @option{-fplugin=@var{name}} instead
@option{-fplugin=@var{path}/@var{name}.so}. This option is not meant
to be used by the user, but only passed by the driver.